Secondary electron conduction electron tube



May 27, 1969 M. GREEN Er AL $447,024

SECONDARY ELECTRON CONDUCTION ELECTRON TUBE Filed June l2, 195'? mvENToRs Morhn Green ond Howard I. Mc DevittJr. BY

r. QM ATTORNE United States Patent O U.S. Cl. 315-12 4 Claims ABSTRACT OF THE DISCLOSURE A system and method of operation of an electronic tube incorporating a secondary electron conduction storage target to provide high speed modulation of the amplification of the storage target.

BACKGROUND OF THE INVENTION This invention relates to electron -devices and methods of operating these devices which incorporate secondary electron conduction (SEC) targets to obtain high speed gain modulation of the secondary electron conduction target.

This invention is generally applicable to secondary electron conduction tubes of the type described in U.S. Patent 3,212,316 and copending application Ser. No. 440,076 filed Mar. 16, 1965, and issued as U.S. Patent 3,419,747, by M. Green and assigned to the same assignee as this invention. The above-mentioned patent describes the general operating characteristics of a secondary electron conduction tube and the above-mentioned patent application describes a specific mode of operation in order to obtain a moving target indication type system. This invention is generally directed to a modified system and method of operation in order to provide a high speed gain modulation.

In a secondary electron conduction tube the primary electrons bombard the secondary electron conduction target causing the liberation of free secondary electrons within the low density insulating layer. The flow of the free electrons to the signal plate and the emission of free electrons to the suppressor mesh located on the opposite side of the target with respect to the source of primary electrons is controlled by the relation of the conductive backplate electrode potential VT and the suppressor mesh electrode potential VM to the potential VS of the exposed target surface facing the suppressor mesh and the reading gun. Normally the storage target is scanned before integration of an input signal to b-ring the exposed surface potential Vs to the same potential as the gun cathode of the read gun which is also ground potential.

The three important operating conditions for a secondary electron conduction tube are as follows with the potentials given below with respect to VS. A iirst condition is with VT positive and VM positive. A large positive secondary elect-ron conduction gain component is obtained in which the free electrons travel to the conductive backplate. There is also a positive `gain component resulting from the secondaries that are emitted from the exposed surface of the target and collected by the suppressor mesh. This latter current is referred to as transmission secondary emission (TSE). The net result is a large positive gain caused by a positive increase in the amount of charge and the value of Vs on the exposed surface of the porous layer. A second condition is with VT negative and VM positive. In this case, the secondary electron conduction gain component is zero, in that free electrons will not flow to the negative backplate elecr'ce trode, but a small positive transmission secondary emission gain will be obtained due to emission of secondary electrons which are collected by the suppressor mesh. There will also be a small negative gain component resulting from the deposition of the primary electrons and the flow of charge into the target from the back plate. The net result is a small positive gain if VS is not too negative and the primary electron energy is not too low. A third condition is lwith VT negative and VM negative. In this case, the secondary electron conduction `gain is zero. The transmission secondary emission gain component is zero and there is a small negative gain component due to the deposition of the primary elect-rons and the flow of charge into the target from the signal plate. The net result is a small negative gain associated with a decrease in the amount of charge and the value of VS on the target surface.

From the above operating conditions, the gain of the target may be controlled by changing either VT or VM with respect to VS. However, in a secondary electron conduction tube of the type described in U.S. Patent 3,213,316 with the mesh spaced at a distance of about 0.0622 inch from the surface, only changes in VM permit significant modilication of the gain of the target during integration, and this technique has important limitations.

`Changes in VT during the integration period are not effective because the capacitance between the charge plane Within the target and the signal plate is large compared With the capacity between the charge plane and the suppressor mesh. Changes in VT produce about equal changes in VS and at least one rscan of the ta-rget iS required to return VS to its original value and alter the difference between VT and VS. Changes in VM can be used to control the instantaneous gain, but they must be made with VT negative if a significant eiect is to be obtained. This restricts the use of the method to changes between small positive and small negative gain values. Furthermore a continuous variation of again is not possible because redistribution of the secondary electrons degrades resolution when the mesh is at a small negative potential. This method of gain control during ntegration does have applications as has been set forth in the above-mentioned patent application.

In a secondary electron conduction camera tube with wide spaced mesh, that is of a distance of about 0.100 inch from the target, there are two important limitations to gain control accomplished by varying the suppressor mesh potential VM. First, there is an inability to alter the difference between VT and Vs and hence the secondary electron conduction gain. As a consequence, if gain control is to be attempted during integration, VT and Vs must be negative to eliminate the SEC gain component. Only a small TSE gain component can be used. Secondly, a resolution degradation by redistn'bw tion of secondary electrons is found when the mesh operates at small negative potentials.

SUMMARY OF THE INVENTION This invention is directed to the use of a closely spaced mesh tube for gain control during integration. By close spaced mesh, this means a spacing of about 0.010 inch or less. In such a secondary electron conduction tube with a closely spaced mesh, the capacitive coupling between the mesh and the charge plane in the target is sucient to enable changes in the potential diierence between Vs and VT to be made during integration. Hence, operation in the secondary electron conduction mode with high `gain is possible, and it is no longer necessary to use transmission secondary emission gain components. This in turn permits the suppressor mesh to be kept negative throughout integration and therefore redistribution problems cannot cause significant loss of resolution and degradation.

It is accordingly an object of this invention to provide an improved secondary electron conduction system for high speed gain modulation.

It is another object of this invention to provide a method of operating a secondary electron conduction tube for high speed gain modulation.

Briefly, the objects of this invention are accomplished by providing a closely spaced suppressor mesh with respect to the exposed surface of a secondary electron conduction target and the application of suitable potentials thereto so that the device operates within the large positive gain region due to free or secondary electron conduction and does not rely on transmission secondary emission for establishing the charge on the exposed surface of the target electrode.

These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is an elevational view in section, schematically representing a pickup tube and associated system in accordance with the teachings of this invention; and

FIG. 2 is an enlarged elevational view in section illustrating the target electrode assembly in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, there is illustrated a pickup tube incorporating the teachings of this invention. The tube includes an evacuated envelope 10. A face plate portion 12 is provided in the envelope 10 and is transmissive to the desired scene radiation. The face plate 12 is of a suitable material such as glass in the case of a visible light input. A coating 14 of a suitable photoemissive material sensitive to the input radiation such as cesium antimony in the case of visible light input is provided on the inner surface of the face plate 12. An electron gun 20 is provided at the opposite end of the envelope for generating and forming a pencil-like electron beam which is directed onto a target assembly 30. The electron gun is referred to as the reading electron gun. The target assembly is positioned between the electron gun 20 and the photocathode 14. Between the target assembly 30 and the photocathode 14, there is provided a plurality of electrodes illustrated as 16 and 18 with suitable potentials provided thereon for accelerating and focusing the photoelectrons emitted from the photocathode 14 onto the target assembly 30. Positioned between the target assembly 30 and the electron gun 20, there is provided a grid member of electrically conductive materials such as nickel which is located at a distance of about 0.010 inch from the surface of the target assembly 30.

The target assembly 30 is comprised of a suitable support ring 32 of suitable material and having a suitable electrically conductive support film 34 such as aluminum attached to the ring 32. The film 34 serves as the signal backplate electrode. A conductive mesh could also be utilized for the support lm 34 and also would serve as the conductive backplate. The porous coating or film 36 is comprised of a suitable insulating material which exhibits the property of the generation of internal secondary electrons in response to electron bombardment of one surface. These internal electrons may be emitted from the opposite surface or permitted to be conducted within the voids of the porous coating 36. The coating 36 may be of any suitable material such as an alkali or alkaline earth metal compound such as potassium chloride, magnesium chloride, or magnesium oxide.

The electrode 40 may have a transmission characteristic of and have about one million apertures per square inch. The electron gun 20 is of any suitable type for producing a low Velocity pencil-like electron beam to be scanned over the surface of the target electrode 30. The electron gun 20 may consist of a cathode 22, a control grid 24 and an accelerating grid 26. The gun electrodes 22, 24 and 26 along with a coating 44 provided on the inner surface of the wall provide a focused electron beam on the target 30. Deection means 50 illustrated as a coil is provided around the envelope 10 for deection of the electron beam generated by the electron gun 20. By application of suitable currents to the coil 50, the electron beam from the gun 20 is scanned over the surface of the target 30 in a suitable manner. A magnetic coil 52 is also provided around the envelope 10 to provide additional focusing of the electron beam from the electron gun 20 onto the target 30 as well as for focusing the electrons from the photocathode 14 onto the target 30. The photocathode 14 is considered the write gun of the assembly.

The target electrode 30 is fully described in the previously mentioned patent. The target 30 consists of the aluminum film 34 of a thickness of about 1000 angstrom units with a porous coating 36 provided on the conductive lm 34. The film 36 is deposited in an inert atmosphere such as argon. A suitable material is potassium chloride. The film 36 has a thickness of approximately 20 microns and has a density less than 10% of its bulk density.

Representative potentials are illustrated in FIGURE 1 applied to the electrode configuration. A modulation source 70 is connected to the grid 40. The modulation source 70 may be of any suitable potential source to provide the necessary potentials to modulate the suppressor grid 40 in order to control the gain.

The device operates in the following manner. Prior to integration and after readout the target backplate is held at a negative potential of 8 volts. The exposed surface of the target is at ground potential. During integration the suppressor mesh potential is varied to give the desired gain. This may be illustrated 'by the following table:

The above values represent only selected examples. Continuous gain variation is possible over the entire range. The typical target mesh spacing for the SEC tube would be 10 mils. The target to mesh capacity for this spacing is 3.5 picofarads per square centimeter. Since the SEC target may have a storage capacity of 70 picofarads per square centimeter, a 20-volt change in mesh potential produces about a l-volt change in the relative potentials of the charge plane in the target and the signal plate. This is illustrated by the table above. Thus, during integration, modulation of the mesh potential is used to control the magnitude and the sign of the gain as indicated in the above table. Measurements have shown that the SEC target has sufficient strength to withstand the electrical elds between the target and the mesh required for this mode of operation. It should also be noted that the mesh is always negative during integration and excessive voltage excursions cannot be developed in the target. This negative potential also prevents resolution degradation by redistribution of secondary electrons.

After integration, the target surface is brought to a positive potential to permit both positive and negative going voltage excursions to be readout. The appropriate change in potential may be accomplished by making either VT or VM positive. For example, before readout assume the target backplate potential to be negative 8 volts, assume VS to be negative 3 volts and the voltage of the mesh to be negative 10 volts. The mesh potential is then shifted from the negative l0 volts to positive 110 volts and this action results in changing the exposed surface potential from a negative 3 volts to a positive 3 volts. After scanning -by the readout beam from the electron gun Z0 the potential of the exposed surface or VS will be at ground potential and after a suitable priming operation integration may again be accomplished.

This system could .be utilized for image processing requiring modulation by a time dependent function. It also could be utilized for high speed shuttering particularly in magnetically focused type camera tubes and it could also be utilized in a range finding with a modulated laser. Various modifications may be made within the spirit of the invention.

We claim as our invention:

1. In combination, a cathode ray tube having a target electrode assembly, said target electrode assembly comprising a conductive layer, a porous layer of storage material provided on said conductive electrode exhibiting the property of transmission secondary electron emission and also secondary electron conduction wherein impingement of electrons on said storage layer generates secondary electrons within said layer which are conducted through the Ypores in the material and may be emitted from the exposed surface or collected by said conductive electrode, means for directing a writing electron beam through said conductive electrode onto one side of said storage layer, means for directing a reading beam onto the opposite side of said storage layer, an electrically conductive mesh positioned adjacent and spaced from said exposed surface of said storage layer facing said reading gun, means for modulating the potential of said electrically conductive mesh with respect to said surface of said storage layer to vary the charging rate of said exposed surface of said storage layer, said modulating potential being negative with respect to said exposed surface at all times during integration of said writing information.

2. The system described in claim 1 in which said electrical conductive mesh is positioned at a distance of about 10 mils from the exposed surface of said storage layer.

3. The system described in claim 1 in which readout of information from said target by said readout gun is accomplished by operating said electrically conductive mesh at a positive potential with respect to the surface of said storage layer.

4. The system described in claim 1 in which said electrical conductive mesh is positioned at a distance of less than 10 mils from the exposed surface of said storage layer.

References Cited UNITED STATES PATENTS 3,094,644 6/ 1963 Buckbee et al 315-12 3,174,071 3/1965 Eberhardt 315-12 3,213,316 10/1965 Goetze et al. 315-12 3,240,988 3/1966 Charles 315-12 RODNEY D. BENNETT, JR., Primary Examiner. JEFFREY P. MORRIS, Assistant Examiner. 

